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Significance Microbes can discriminate among metabolites that differ only in the stable isotopes of the same element. This stable isotope fractionation responds systematically to environmental variables like extracellular metabolite concentrations and to physiological ones like cell-specific metabolic rates. These observable characteristics define a stable isotope phenotype, as exemplified by the rich database of experimental sulfur isotope fractionations from sulfate-respiring bacteria and archaea. We developed a quantitative model for sulfur isotope fractionation during sulfate respiration that incorporates only experimentally accessible biochemical information. With this approach, stable isotope phenotypes can be decomposed into their physiological, enzymatic, and environmental parts, potentially illuminating the relative influences of these components in natural microbial populations today, as well as how they may have varied in the deep past. We present a quantitative model for sulfur isotope fractionation accompanying bacterial and archaeal dissimilatory sulfate respiration. By incorporating independently available biochemical data, the model can reproduce a large number of recent experimental fractionation measurements with only three free parameters: ( i ) the sulfur isotope selectivity of sulfate uptake into the cytoplasm, ( ii ) the ratio of reduced to oxidized electron carriers supporting the respiration pathway, and ( iii ) the ratio of in vitro to in vivo levels of respiratory enzyme activity. Fractionation is influenced by all steps in the dissimilatory pathway, which means that environmental sulfate and sulfide levels control sulfur isotope fractionation through the proximate influence of intracellular metabolites. Although sulfur isotope fractionation is a phenotypic trait that appears to be strain specific, we show that it converges on near-thermodynamic behavior, even at micromolar sulfate levels, as long as intracellular sulfate reduction rates are low enough (<<1 fmol H ₂S⋅cell ⁻¹⋅d ⁻¹).

We have developed a technique for cultivation of chemolithoautotrophs under high hydrostatic pressures that is successfully applicable to various types of deep-sea chemolithoautotrophs, including methanogens. It is based on a glass-syringe-sealing liquid medium and gas mixture used in conjunction with a butyl rubber piston and a metallic needle stuck into butyl rubber. By using this technique, growth, survival, and methane production of a newly isolated, hyperthermophilic methanogen Methanopyrus kandleri strain 116 are characterized under high temperatures and hydrostatic pressures. Elevated hydrostatic pressures extend the temperature maximum for possible cell proliferation from 116°C at 0.4 MPa to 122°C at 20 MPa, providing the potential for growth even at 122°C under an in situ high pressure. In addition, piezophilic growth significantly affected stable carbon isotope fractionation of methanogenesis from CO₂. Under conventional growth conditions, the isotope fractionation of methanogenesis by M. kandleri strain 116 was similar to values (-34[per thousand] to-27[per thousand]) previously reported for other hydrogenotrophic methanogens. However, under high hydrostatic pressures, the isotope fractionation effect became much smaller (<-12[per thousand]), and the kinetic isotope effect at 122°C and 40 MPa was -9.4[per thousand], which is one of the smallest effects ever reported. This observation will shed light on the sources and production mechanisms of deep-sea methane.

The disappearance of mass-independent sulfur isotope fractionation (S-MIF) within the c. 2.3-billion-year-old (Ga) Rooihoogte Formation has been heralded as a chemostratigraphic marker of permanent atmospheric oxygenation. Reports of younger S-MIF, however, question this narrative, leaving significant uncertainties surrounding the timing, tempo, and trajectory of Earth’s oxygenation. Leveraging a new bulk quadruple S-isotope record, we return to the South African Transvaal Basin in search of support for supposed oscillations in atmospheric oxygen beyond 2.3 Ga. Here, as expected, within the Rooihoogte Formation, our data capture a collapse in Δ3×S values and a shift from Archean-like Δ36S/Δ33S slopes to their mass-dependent counterparts. Importantly, the interrogation of a Δ33S-exotic grain reveals extreme spatial variability, whereby atypically large Δ33S values are separated from more typical Paleoproterozoic values by a subtle grain-housed siderophile-enriched band. This isotopic juxtaposition signals the coexistence of two sulfur pools that were able to escape diagenetic homogenization. These large Δ33S values require an active photochemical sulfur source, fingerprinting atmospheric S-MIF production after its documented cessation elsewhere at ∼2.4 Ga. By contrast, the Δ33S monotony observed in overlying Timeball Hill Formation, with muted Δ33S values (<0.3) and predominantly mass-dependent Δ36S/Δ33S systematics, remains in stark contrast to recent reports of pronounced S-MIF within proximal formational equivalents. If reflective of atmospheric processes, these observed kilometer-scale discrepancies disclose heterogenous S-MIF delivery to the Transvaal Basin and/or poorly resolved fleeting returns to S-MIF production. Rigorous bulk and grain-scale analytical campaigns remain paramount to refine our understanding of Earth’s oxygenation and substantiate claims of post-2.3 Ga oscillations in atmospheric oxygen.

Cold seeps are unique deep‐sea ecosystems that play an important role in marine biogeochemical cycles. However, the dynamics and regulation of sulfur isotope biogeochemistry in seep sediments remain unconstrained. We investigated the geochemical characteristics of active seep sites, including the content and stable isotope composition of sulfur species and sulfate reduction (SR) rates. The S‐shaped downcore distribution of sulfate suggested non‐steady state diagenesis induced by lateral advection of venting fluids. 34S‐enriched pyrite (−7.3‰) was found in surface sediment, suggesting the low sulfur isotope fractionation, calculated between 30 and 35‰. High estimates of cell‐specific sulfate reduction rates, 18–69 fmol cell−1 day−1, together with data collected from previous studies, confirmed that elevated SR activity at seeps contributed to smaller sulfur isotope fractionation (11.5–35‰) than found typically in shelf sediments (44.5–66.5‰), leading to 34S‐enriched pyrite (∼–10‰). These results provided insights into sulfur isotope biogeochemistry in cold seep sediments.

The upper mantle is heterogeneous in Fe isotope compositions, but the origin of the heterogeneity needs understanding. Recent studies on oceanic basalts demonstrate that the upper mantle Fe isotope heterogeneity results from low‐degree melt metasomatism. However, whether this hypothesis is of global significance needs testing in continental settings. Here we present an Fe isotopic study on intraplate continental alkali basalts from continental China, which are shown to have derived from upper mantle containing more metasomatized lithologies. The results show first‐order positive correlations of δ56Fe with indices of low‐degree melt metasomatism (e.g., [La/Sm]N), which, together with the so‐far published mid‐ocean ridge basalts data, define global trends, substantiating the global significance of low‐degree melt metasomatism in causing upper mantle Fe isotope variation. We, thus, suggest that low‐degree melt metasomatism is a globally effective process to preferentially concentrate heavy Fe isotopes in metasomatic lithologies at upper mantle conditions both today and in Earth's history. Plain Language Summary Previous studies have demonstrated that the upper mantle of the Earth has a heterogeneous iron isotope composition, but the cause to such a heterogeneity remains not well understood. By studying oceanic basalts from Mid‐Atlantic Ridge and East Pacific Rise, we suggest that oceanic upper mantle heterogeneous in iron isotope is the result of the mantle metasomatism, which is an enrichment process by low‐degree partial melt at upper mantle condition that progressively enrich more incompatible trace element as well as heavy iron isotopes (57,56Fe vs. 54Fe). However, whether this deep mantle process is globally widespread needs further testing using continental basalts. We here present an iron isotope study on intraplate alkali basalts from eastern continental China. The results are in line with the hypothesis based on the oceanic basalts, substantiating the global significance of melt metasomatism‐induced upper mantle iron isotope fractionation. We further suggest that this deep mantle process fractionate iron isotopes through preferentially concentrating heavy Fe isotopes in metasomatic lithologies at upper mantle conditions, both in Earth's history and at present. Key Points Iron isotope study on continental alkali basalts verifies the global significance of low‐degree melt metasomatism in causing upper mantle Fe isotope fractionation Low‐degree melt metasomatism is a globally effective process to concentrate heavy Fe isotopes at upper mantle conditions The low‐degree melt metasomatism associated Fe isotope fractionation could be both recent and ancient

To better investigate the behavior of titanium (Ti) isotopes during magmatic processes, we report high-precision Ti isotope compositions for 60 terrestrial igneous rocks from different geological settings worldwide. Based on their major element compositions and petrographic descriptions, these samples can be subdivided into two groups: Fe-Ti oxide unsaturated and Fe-Ti oxide saturated. The Fe-Ti oxide unsaturated group samples show a narrow δ49/47Ti (δ49/47Ti = [(49Ti/47Ti)sample/(49Ti/47Ti)OL-Ti] × 1000) range (− 0.036 ± 0.043‰ to 0.082 ± 0.021 ‰), and no correlation between δ49/47Ti and the degree of differentiation is observed. By contrast, Fe-Ti oxide saturated group samples show a remarkable δ49/47Ti variation, ranging from 0.005 ± 0.018‰ to 1.914 ± 0.006 ‰, which are positively correlated with SiO2 contents, and negatively correlated with MgO contents. In particular, multiple SiO2 vs. δ49/47Ti trends are observed in Fe-Ti oxide saturated group, which are controlled by crystal fractionation degrees, magma SiO2 compositions, and Fe-Ti oxide compositions during magma differentiation.

The gas in-place (GIP) content and the ratio of adsorbed/free gas are two key parameters for the assessment of shale gas resources and have thus received extensive attention. A variety of methods have been proposed to solve these issues, however none have gained widespread acceptance. Carbon isotope fractionation during the methane transport process provides abundant information, serving as an effective method for differentiating the gas transport processes of adsorbed gas and free gas and ultimately evaluating the two key parameters. In this study, four stages of methane carbon isotope fractionation were documented during a laboratory experiment that simulated gas transport through shale. The four stages reflect different transport processes: the free gas seepage stage (I), transition stage (II), adsorbed gas desorption stage (III) and concentration diffusion stage (IV). Combined with the results of decoupling experiments, the isotope fractionation characteristics donated by the single effect (seepage, adsorption-desorption and diffusion) were clearly revealed. We further propose a technique integrating the Amoco curve fit (ACF) method and carbon isotope fractionation (CIF) to determine the dynamic change in adsorbed and free gas ratios during gas production. We find that the gases produced in stage I are primarily composed of free gas and that carbon isotope ratios of methane (δ 13 C 1 ) are stable and equal to the ratios of source gas (δ 13 C 0 1 ). In stage II, the contribution of free gas decreases, while the proportion of adsorbed gas increases, and the δ13C1 gradually becomes lighter. With the depletion of free gas, the adsorbed gas contribution in stage III reaches 100%, and the δ 13 C 1 becomes heavier. Finally, in stage IV, the desorbed gas remaining in the pore spaces diffuses out under the concentration difference, and the δ 13 C 1 becomes lighter again and finally stabilizes. In addition, a kinetic model for the quantitative description of isotope fractionation during desorption and diffusion was established.

In terrestrial magmas titanium is predominantly tetravalent (Ti 4+ ), in contrast, lunar magmas are more reduced (IW-1) and hence approximately 10% of their bulk Ti content is trivalent (Ti 3+ ). Changes in oxidation state and coordination number are both important parameters that can serve to drive Ti stable isotope fractionation. As such, mineral–mineral and mineral-melt Ti stable isotope fractionation factors determined for terrestrial samples may not be appropriate for lunar samples that formed under more reducing conditions. To address this issue, several experiments were carried out in gas mixing furnaces over a range of f O 2 (air to IW-1) to determine Ti stable isotope fractionation factors for minerals, such as ilmenite, clinopyroxene and rutile that are highly abundant on the Moon. Results show that the extent of Ti stable isotope fractionation significantly increases with decreasing f O 2 . For example, the isotopic difference between ilmenite and residual melt (Δ 49 Ti ilmenite-melt ) is resolvably lower by ~ 0.44 ‰ from terrestrial-like FMQ-0.5 to lunar-like IW-1 at an intermediate precision of ± 0.003 ‰ (95% c.i. OL–Ti). This confirms that fractionation factors determined for terrestrial conditions are indeed not applicable to lunar settings. Our new fractionation factors for ilmenite, clinopyroxene and silicate melt are mostly consistent with those previously determined by ab initio modelling based on density-functional theory. Using our new experimental data in conjunction with previously published high-precision HFSE data and Ti stable isotope data of lunar basalts, we modelled the solidification of the Lunar Magma Ocean (LMO). The model for LMO solidification included fractionation of Ti stable isotopes not only by Ti-oxides, but also by typical lunar silicate minerals as pyroxene or olivine. The resulting δ 49 Ti for urKREEP and ilmenite-bearing cumulates are within error of previous estimates, but also indicate that ilmenite-bearing cumulates must have contained around 15% ilmenite.

A redox reaction in which Sn2+ oxidizes to Sn4+ is thought to occur during the precipitation of cassiterite (SnO2) and stannite (Cu2FeSnS4) from high-temperature hydrothermal solutions. In four stanniferous regions with differing mineralization environments (South Dakota, U.S.A.; Cornwall, England; Erzgebirge, Germany/Czech Republic; Andean tin belt, Bolivia), the tin isotope composition in stannite (mean value δ124Sn = -1.47 ± 0.54 ppm, n = 21) is consistently more fractionated toward negative values than that of paragenetically earlier cassiterite (mean value δ124Sn = 0.48 ± 0.62 ppm, n = 50). Given the oxidation-dependent mechanism for cassiterite precipitation, this isotopic shift is most likely attributable to the oxidation of Sn in solution; precipitation of heavy-Sn-enriched cassiterite results in residual dissolved Sn with lighter isotopic composition, which is expressed in the negative δ124Sn values of later-formed stannite. Equally important is that the mean values for the cassiterite from the various deposits are slightly different and may indicate that the initial Sn isotope composition in early-formed cassiterite relates to variations in the source or magmatic processes. Therefore, the Sn isotopes may provide information on both redox reactions and petrologic sources and processes.

High-precision cadmium (Cd) isotope compositions are reported for sphalerite, galena, and smithsonite from the Fule Zn–Pb–Cd deposit, a typical Mississippi Valley-type deposit in Southwest China. Dark sphalerite has lighter δ 114/110 Cd values (0.06 to 0.46 ‰) than light sphalerite (0.43 to 0.70 ‰), and the Cd in galena is primarily in the form of sphalerite micro-inclusions with δ 114/110 Cd of −0.35 to 0.39 ‰. From early to late stages, δ 114/110 Cd values of smithsonite regularly increase from 0.19 to 0.42 ‰, whereas Cd/Zn ratios decrease from 252 to 136; the δ 114/110 Cd variation pattern of supergene smithsonite reflects kinetic Rayleigh fractionation during low-temperature processes. From the bottom to the top of the orebody, the dark sphalerite has different patterns in δ 114/110 Cd values, Cd/Zn ratios, δ 34 S values, and Fe concentrations compared to the light sphalerite, indicating that dark and light sphalerite formed by different processes. The varying patterns of δ 144/110 Cd values and Cd/Zn ratios within light sphalerite are similar to those of layered smithsonite, and the δ 144/110 Cd values have a positive correlation with δ 34 S values, indicating that Cd isotope fractionation in the light sphalerite was due to kinetic Rayleigh fractionation. Instead, in dark sphalerite, the δ 144/110 Cd values have a negative correlation with δ 34 S values and a positive correlation with the Cd/Zn ratio. Thus, it can be concluded that dark sphalerite could be modeled in terms of two-component mixing (basement fluid and host-rock fluid), which is in agreement with previous explanations for the negative correlation between δ 66 Zn and δ 34 S in some typical Zn–Pb deposits. We propose that the significant variation in Cd isotope composition observed in the Fule Zn–Pb–Cd deposit confirms that Cd isotopes can be used for tracing fluid evolution and ore formation.